It would seem that the 50th Democratic vote in the Senate -- and I don't know who exactly that is, probably some Southerner -- would have much to gain by the de-emphasis of the filibuster. If legislation needed only 50 votes to pass the Senate, people designing stimulus bills and health care plans and energy legislation would take that into account. They wouldn't have to sell their bill to Ben Nelson, and they'd write more ambitious things. Figuring out some way to tailor bills to Senator Fifty, or else buy Senator Fifty's vote with a variety of shiny trinkets, would be the goal. You can see how Senator Fifty might be quite pleased with this dynamic.
But I haven't seen any evidence that potential Senator Fifties are acting on these incentives. You don't see, say, Bob Casey or Mark Pryor or Mary Landrieu or Kay Hagan dissing on the filibuster and calling out Ben Nelson for obstructionism. And I don't really know why. Obviously, there are going to be a number of people jockeying to be Senator Fifty, but in a session where people are trying to move lots of historic legislation, the stock of rewards is probably large enough that everybody in the 45-55 range has a reasonable shot at some share of the goodies. So why is everybody letting Ben Nelson have all the fun?